Origin of bedding-parallel fibrous calcite veins in lacustrine black shale: A case study from Dongying Depression, Bohai Bay Basin

Marine and Petroleum Geology 102 (2019) 873–885

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Research paper

Origin of bedding-parallel fibrous calcite veins in lacustrine black shale: A case study from Dongying Depression, Bohai Bay Basin

T

Guoqiang Luana, Chunmei Donga,b,∗, Karem Azmyc, Chengyan Lina,b, Cunfei Maa, Lihua Rena,b, Zhaoqun Zhud a

School of Geosciences, China University of Petroleum (East China), Qingdao, 266580, China Reservoir Geology Key Laboratory of Shandong Province (East China), Qingdao, 266580, China c Department of Earth Sciences, Memorial University of Newfoundland, St John's, Newfoundland, A1B 3X5, Canada d School of Earth Science and Engineering, Hebei University of Engineering, Handan, 056038, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Fibrous calcite veins Black shale Force of crystallization Eocene Dongying Depression

The bedding-parallel fibrous calcite veins in black shales are common in sedimentary basins and retain significant information about shale diagenesis and organic matter evolution. However, the origin of the beddingparallel fibrous calcite veins in black shales is still of great controversy. Carbonaceous black shales of Es4s–Es3x interval are the main source rock in Dongying Depression, Bohai Bay Basin, and fibrous calcite veins parallel to bedding are widely spread in these shales. Petrographic examination, fluid-inclusion microthermometry, and isotopic analyses were conducted to study the timing, diagenetic fluid conditions and formation mechanism of fibrous calcite veins. The bedding-parallel fibrous calcite veins include beef veins and cone-in-cone structures. The beef veins occur as short lenses and are filled with sub-vertical fibrous calcite. They contain a “median line” defined with brown granular calcite, scattered host rock fragment and pyrite framboids. Several adjacent beef veins may combine and grow together as a cone-in-cone structure and black shale laminas involved in it occur as sinusoidal solid inclusions in slices. Primary two-phase inclusions in fibrous calcite have homogenization temperatures (Th) between 86.4 °C and 117.4 °C. The δ13C composition of the micritic calcite in host shale ranges from +3.7‰ to +6.3‰VPDB and that of granular calcite ranges from −0.2‰ to +1.4‰VPDB, but the fibrous calcite has a moderate δ13C values ranging from +1.8‰ to +5.0‰ VPDB. The δ13C compositions of these calcite suggest an evolving carbon source at different burial stages. The fibrous calcite likely precipitated from modified pore water at elevated temperature, which is supported by its low δ18O values (−13.7‰ to −11.4‰ VPDB). The veins formed over two stage and the fibrous calcite growth was continuous and at least partly driven by the force of crystallization. The bicarbonate responsible for the fibrous calcite was derived from a mixed source including inorganic carbon from previous carbonate dissolution, and organic carbon from both fermentation and thermal decarboxylation.

1. Introduction Fibrous calcite veins (including beef veins and cone-in-cone structures) parallel to bedding planes are common in some sedimentary basins, especially in black shales that are rich in organic matter and carbonates (Rodrigues et al., 2009; Tribovillard et al., 2018). They contain valuable information about the nature of fluid flow in sedimentary basins, the sources of cement material, and the geochemical evolution of the pore waters during the burial diagenetic history of the host shales (Marshall, 1982; Curtis et al., 1986; Thyne and Boles, 1989; Coniglio et al., 1990; Morad and Eshete, 1990; Vrolijk and Sheppard, ∗

1991). Although the veins have been globally reported (Cobbold et al., 2013), the formation mechanism is still controversial. Based on petrographic, fluid inclusion and isotopic and elemental geochemical studies, the fibrous calcite have been ascribed to both early and late diagenetic processes in marine and burial environments. Some workers suggest that the veins occurred at shallow to intermediate burial with depths ranging from tens to hundreds of meters (Franks, 1969; Mackenzie, 1972; Al-Aasm et al., 1993; Al-Aasm et al., 1995; Meng et al., 2017; Tribovillard et al., 2018), whereas others advocate that they form at the “oil window” (Marshall, 1982; Rodrigues, 2008; Zanella and Cobbold, 2011, 2012; Cobbold et al.,

Corresponding author. School of Geosciences, China University of Petroleum (East China), Qingdao, 266580, China. E-mail address: [email protected] (C. Dong).

https://doi.org/10.1016/j.marpetgeo.2019.01.010 Received 30 July 2018; Received in revised form 7 January 2019; Accepted 8 January 2019 Available online 10 January 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved.

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overpressure first occurred possibly during deposition of Es3, because of the high sedimentation rate (up to 500 m/m.y), and continued until the end of Ed with the maximum residual pressure of 10 MPa (overpressure coefficient from 1.2 to 1.5). The compaction disequilibrium was likely the main mechanism of the overpressure during Es3—Ed (Bao et al., 2007; Guo et al., 2010; Yang et al., 2014; Meng et al., 2017). Pressure release by tectonic uplift occurred at the end of the Ed and led to the first phase of hydrocarbon migration (Zhu, 2004; Bao et al., 2007). Residual pressure increased slowly during the deposition of Ng Formation, but was much lower than present overpressure. The present overpressure system with the maximum residual pressure of 20 MPa was mainly formed during Nm-Qp when great deal of hydrocarbon was generated from the source rock of Es4s–Es3x (Li et al., 2008; Guo et al., 2010).

2013; Zanella et al., 2014, 2015). Bicarbonates needed for the precipitation of fibrous calcite were mainly derived from connate waters or dissolution of the marine carbonates originally present in great abundance within the shale, and partly from decomposition of organic matters (Al-Aasm et al., 1993, 1995; Meng et al., 2017). The most generally accepted explanation for the opening mechanism of fibrous veins is the crack-seal model originally proposed by Ramsay (1980). Many researchers have attributed this mechanism to overpressure (AlAasm et al., 1995; Parnell, 1995; Sellés-Martínez, 1996; Oliver and Bons, 2001; Basson and Viola, 2004). When fluid pressure exceeds the overburden stress, the vertical effective stress becomes tensile and results in horizontal fractures (Cobbold and Rodrigues, 2007). An alternative scenario for the widening of fibrous veins favors the force of crystallization (Fletcher and Merino, 2001; Means and Li, 2001; Wiltschko and Morse, 2001), which provides the necessary strength for fibres to grow incrementally without fractures being created during growth (Bons and Jessell, 1997). The fibrous calcite veins (including beef veins and cone-in-cone structures) are common in the Eocene shale from Dongying Depression of the Bohai Bay Basin, East China. It has been suggested that the initiation of calcite veins exposed in the study area was by the overpressure due to hydrocarbon generation (Zhang et al., 2016). The dissolution of carbonate in black shale induced by organic acids was believed to be the source for calcite fibres (Wang et al., 2005). The main objective of the current study is to integrate petrography, fluid inclusions, and stable isotopes to constrain the timing, diagenetic fluid conditions and formation mechanism of fibrous calcite veins in Eocene black shale from Dongying Depression.

3. Methods The shales with fibrous calcite veins were sampled from 3 cores (Fig. 1B) in the study area. Thin sections were prepared for petrographic examination under a microscope with an attached video camera system. Cathodoluminenscence was performed on polished thin sections to identify the cement generations by using CITL's CL8200 Mk5-2 model system, at 14–15 kV and 300–350 mA, attached to a microscope with a camera system. Microthermometric analysis was conducted on wafers (∼80 μmthick) from fibrous calcite using a LINKAM THMSG600 heating–freezing stage. Homogenization temperature (Th) values of primary two-phase fluid inclusions (Goldstein and Reynolds, 1994) were measured using a heating rate of 10 °C/min (18 °F/min) at temperatures less than 80 °C (176 °F) and a rate of 5 °C/min (9 °F/min) at temperatures exceeding 80 °C (176 °F). For carbon- and oxygen-isotope analyses, fibrous calcite and brown granular calcite were sampled by a microscope-mounted drill assembly, whereas the host rock shale samples (without veins) were powdered. The powder was reacted in inert atmosphere with ultrapure orthophosphoric acid at 25 °C. The extracted CO2 was carried by helium through chromatographic column and transferred to Thermo Finnigan DELTA V Plus isotope ratio mass spectrometer, in which the gas was ionized and measured for isotopic ratios. All analyses were converted to VPDB and corrected for O17 in accordance with the procedure outlined by Craig (1957). Uncertainties of better than 0.15‰ (2α) for the analyses were determined by repeated measurements of GBW04405 (δ13C = +0.57‰ and δ18O = −8.49‰ vs. VPDB) and (δ13C = −10.85‰ and δ18O = −12.40‰ vs. VPDB) as well as internal standards.

2. Geologic setting Dongying depression is a typical lacustrine half-graben located in the southeastern part of the Bohai Bay Basin in East China (Fig. 1A). It covers an area of 5700 km2 (2200 mi2) and bordered by Chenjiazhuang Salient and Binxian Salient in the north; the Luxi Uplift and Guangrao Salient in the south; and Qingcheng Salient in the west; and the Qingtuozi Salient in the east (Fig. 1B). It contains Paleogene, Neogene, and Quaternary sediments with local thicknesses of up to 5000 m (16,400 ft) in the center, consisting of the Kongdian, Shahejie (Es), Dongying, Guantao, Minghuazhen, and Pingyuan formations (Fig. 2). The Es Formation is divided into four members, Es1–Es4 from youngest to oldest. Expansion of the basin started after the Mesozoic and reached maximum during the deposition of Es4s–Es3x relatively deep (> 50 m) lake under warm and humid paleoclimate conditions (Zhu et al., 2005). A series of fans and braided deltas developed along the lake margin during that time and the center of the lake was filled with thick deep water black shales (∼400 m thick), which are the primary source rocks for the petroliferous depression. The Es4s–Es3x interval consists of dark brown, organic-rich laminated shale and massive mudstone with total organic content (TOC) from 0.58 to 11.4 wt % (Zhang et al., 2016). The organic matter is predominantly planktonic algae (e.g., ditch whip algae, coccoliths, and Bohai algae; Wang, 2012). The shale is thermally mature, which is supported by vitrinite reflectance (Ro) measurements (0.46%–0.74%) and Tmax values (430 °C–450 °C) (Liang et al., 2017). Unlike the typical terrigenous shale (> 75% clays; Slatt and Rodriguez, 2012), the shale of Es4s–Es3x consists predominately of carbonate minerals (avg. 59.6%, meanly micritic calcite) followed by clay minerals (avg. 23.5%), quartz and feldspars (avg. 20.6%), and pyrite (avg. 3.9%) (He et al., 2017; Liang et al., 2017). Widespread overpressures (formation pressure > hydrostatic pressure) are present in the Eocene Es3 and Es4 intervals in the depression, with pressure coefficient up to 1.99 from drillstem tests (Guo et al., 2010). Though the mechanism of the overpressure is still a subject of intensive debate, both compaction disequilibrium and hydrocarbon generation are believed to be the viable causes (Hao et al., 1995). The

4. Results 4.1. Petrography of fibrous calcite veins The fibrous calcite veins including beef veins and cone-in-cone structures are bedding-parallel and abundant in organic-rich laminated shale. The veins show a wide range in shape, from long (decimetermeter) thin sheets (Fig. 3A) to short (millimeter-centimeter) lenses (Fig. 3C and D) and occurred in the lamina seam (1–2 cm wide). For a single beef vein, the center or one side edge usually displays a median line, defined by brown fine-grained calcite grains, host rock particles and framboidal pyrite (Fig. 4, Fig. 6C). Brown granular calcite crystals in the “median line” are continuous with fibrous crystals (Fig. 4B). Individual pyrite framboids are regularly shaped, with a diameter ranging from 0.8 μm to 10 μm (Fig. 4C). Fibrous calcite crystals are optically continuous across the median line and oriented approximately perpendicular to the vein wall (Fig. 4A and B). Individual calcite fibres with smooth crystal boundary are 10 μm–200 μm wide and 30 μm–2000 μm long. The cone-in-cone structures contain abundant solid inclusions of the 874

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Fig. 1. Location map of the study area showing (A) the sub-tectonic units of the Bohai Bay Basin and (B) structural features of the Dongying Depression and the sampling well locations in the study area.

has a wide range of sizes, from several to hundred micrometers. Some granular calcites occur as a lenticular aggregates while others as a line (Fig. 7F).

host rock fragments (Fig. 5). Unlike the inclusion bands and inclusion trails aligned parallel to the vein-wall interface (Ramsay, 1980; Cox and Etheridge, 1983; Cox, 1987; Fisher and Brantley, 1992), the solid inclusions vary in size from a few micrometers or less to ∼ 1 cm and are either wedge-like inclusions curved into the vein or arranged in parabolically-shaped loops. Sinusoidal solid inclusions clearly experienced plastic deformation and if traced down they would fit in detail to the parallel undeformed lamina in the host rock (Fig. 5A and B). Most of solid inclusions aligned oblique to the vein wall (Fig. 5A) while some of them are laterally continuous with laminas of the host rock (Fig. 5B). These nearly continuous sinusoidal solid inclusions divide the fibrous calcite into multiple parts (Fig. 5D) and outline the cone-in-cone structure (Franks, 1969; Cobbold et al., 2013). Some pyrites were wrapped in veins together with the laminas (Fig. 4D). Under ultraviolet, the solid inclusions show orange yellow fluorescence (Fig. 6B). In CL mode the veins show a well-defined banding due to discontinuities in CL intensity and color. The calcites fibers, on both sides of the median line, display mostly bright yellow luminescence whereas fine calcite grains in the median line show dull orange luminescence (Fig. 7). Brown granular calcite with layered or random distribution occurs near the fibrous calcite veins (Fig. 7D–F). The granular calcite

4.2. Fluid inclusions microthermometry The primary fluid inclusions (Fig. 9A–D) in the fibrous calcite (Goldstein and Reynolds, 1994), which mostly have a shape of triangle and are smaller than 6 μm, are dominantly two-phase inclusions with a dominating aqueous liquid and 3–10 vol% vapor, occurring individuals. The two-phase inclusions show a wide range of Th from 86.4 °C to 117.4 °C (Table 1 and Fig. 9E). The melting points (Tm ice) data ranges from −7.7 °C to −4.6 °C and the calculated diagenetic fluid salinity has a range of 7.6–11.2 wt% NaCl equivalent (Fig. 9F). Primary hydrocarbon inclusions with yellow-green fluorescence and shape of polygon were also observed in the calcite fibres with random distribution (Fig. 8A–D). 4.3. Stable isotope geochemistry Stable carbon and oxygen isotopic compositions of calcite in veins 875

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(+3.49 ± 1.13‰VPDB), and δ18O values from −13.7‰ to −11.4‰VPDB (−12.67 ± 0.68‰VPDB), while the granular calcite have δ13C values ranging from −0.2‰ to +1.4‰VPDB (0.65 ± 0.70‰VPDB), and δ18O values from −8.4‰ to −7.7 ‰VPDB (−8.05 ± 0.31‰VPDB). The δ13C values of micritic calcite in host rock ranges from +3.2‰ to +6.3‰VPDB (+4.75 ± 0.82‰VPDB) and the δ18O values range from −10.1‰ to −7.2 ‰VPDB (−8.1 ± 0.59‰VPDB). The δ13C signatures of the fibrous calcite veins are very similar to those of micritic calcite in host rock but slightly lighter, while the δ18O signatures are more depleted. The δ18O signatures of the granular calcite are similar to those of the micritic calcite in host rock, while the δ13C signatures are much more depleted. 5. Discussion 5.1. Timing of calcite veins The calcite veins in black shale have been extensively described in the literature, and oil generation has been considered the common inducer (Cobbold et al., 2013; Zhang et al., 2016). Several lines of evidence from petrography and fluid inclusions shed the light on the timing of the calcite fibrous veins in this study: (1) lamina and the solid inclusions in veins were contorted without undergoing brittle rupture suggesting that the veins formed in partially consolidated sediments (Figs. 3B, 4D and 5); (2) Some veins were truncated by later fractures filled with bitumen suggesting that the time of calcite veins formation is earlier than primary migration of hydrocarbon (Fig. 6C and D); (3)The pyrite framboids occurred in the median line and in the solid inclusion suggesting that the granular calcite formed during but the fibrous calcite occurred after bacterial sulphate reduction; (4)The hydrocarbon inclusions trapped in the calcite fibres indicate that the veins formed mainly within the oil window (Fig. 8A–D; Parnell, 1995; Parnell et al., 2000); and (5) Th of primary two-phase fluid inclusions show that the fibrous calcite veins precipitated at 86.4°C–117.4 °C, i.e., within the oil window. In summary, the fibrous calcite veins finally formed at the early stage of oil window while the brown granular calcite acting as seeds of the veins precipitated earlier at shallow part during the bacterial

Fig. 2. Generalized Ceinozoic-Quaternary stratigraphy of the Dongying Depression.

(fibrous calcite and granular calcite) and the whole host rock (micritic calcite) were measured (Table 2 and Fig. 10). Fibrous calcites in veins have δ13C values ranging from +1.8‰ to +5.0‰VPDB

Fig. 3. Photographs of calcite veins in laminated black shale. (A) Platelike vein with abundant solid inclusions, Well Ny1, 3410.6 m. (B) Details of solid inclusions in the part enclosed by a black dotted line frame in A. (C) Lenticular veins with an eogenetic collophanite nodule in it, Well N872, 3206.2 m. (D) Lenticular bedding-parallel veins in black shale, Well N872, 3204.7 m.

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Fig. 4. The thin-section images showing microscopic characteristics of fibrous calcite veins. (A) A fibrous calcite vein with brown median line and clear calcite fibers on both side. Well N38, 3358 m. (B) Details of the median line in (A) having brown fine granular calcite with host rock fragments. The granular calcite is continuous with the fibrous calcite and the dividing line is relatively uniform. (C)The center of the veins displays a “median line” with brown granular calcite and pyrite. Massive pyrite appears in shale matrix. Well NY1, 3375.1 m. (D)The vein contain complex solid inclusions with a wavy undulating form. The inclusion at the lower edge of the vein can be tracked into black shale as a lamina. Pyrite, once in shale, was involved in veins along with the laminas. Well NY1, 3410.6 m. (E) The vein is bent around the eogenetic collophanite nodule because of the difference of the compaction resistance with shale matrix. Well N872, 3206 m. (F) A bioclast included in the vein, Well N872, 3606.2 m. fiCal = fibrous calcite, grCal = granular calcite. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5. Microphotograph of solid inclusions in calcite veins. (A) The wedge-like inclusions occurred on the upper part of the veins and none of them crossed the median line, indicating that the median line formed fist. Well N38, 3363 m. (B) The solid inclusions (yellow arrow) on the margin of the vein can be tracked into host rock. The solid inclusions (red arrow) internal of the vein retained original appearance of lamina unless a little separated by growing calcite fibres. Well NY1, 3410.6 m. (C) The fibrous calcite outline by yellow dotted line is clearly separated from the whole vein, suggesting that the veins containing solid inclusions are actually assemblages of several single veins. Well NY1, 3297.4 m. (D) At least 8 single veins can be identified in the platelike combined veins. The host rocks between single veins were deformed into sinusoidal solid inclusions. Well N872, 3200.34 m. fiCal = fibrous calcite, grCal = granular calcite. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 6. Microphotographs of calcite veins under transmitted light (A, C) and UV light (B, D). (A) and (B) Pyrite embedded in granular calcites, and fibrous calcite with solid inclusions occurred on two side of them. There is no fluorescence in granular calcite, while in fibrous calcite the solid inclusions show yellowish brown fluorescence. Well NY1, 3410.6 m. (C) and (D) fibrous calcite grow on one side of granular calcite, and a fracture with bitumen cut through the vein. Well NY1, 3297.4 m. fiCal = fibrous calcite, grCal = granular calcite. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7. Microphotographs of calcite veins under transmitted light (left) and CL (right). (A) and (B) A fibrous veins with straight duty median line. The median line shows dull orange luminescence while the fibrous calcite shows bright yellow luminescence. Well N38, 3358 m. (C) and (D) Duty granular calcite with dull orange luminescence accounts for half of the vein in the center of the picture. Note the position of the arrow, median line can be out of range of veins. Well N872, 3200.34 m. (E) and (F) Single veins with small part of fibrous calcite on the two side of thick median lines. Note the part in the white dotted frame, great many of small straight calcite line with the same dull orange luminescence with the median line occurred and can be inferred as seeds of the fibrous calcite. Well N872, 3200.34 m. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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process of dissolution and re-crystallization of lacustrine sedimentary carbonate in host rock can't yield large differences in the carbon isotope ratios (Myrttinen et al., 2012), so that it leads to HCO3− with a positive carbon isotopic compositions (+3.2‰ to +6.3‰VPDB). The δ13C composition of the fibrous calcite falls within and slightly below the range of the host rock, which suggests that carbonate ions needed for the precipitation of fibrous calcite were mainly derived from lacustrine sedimentary carbonate originally present in great abundance within the shale. However, the contribution of biocarbonates from the organic matter could not be excluded since part of the fibrous calcite shows a relatively depleted carbon isotopic composition. The negative shift in δ13C values of the granular calcite indicates that more 13C-depleted carbon from the organic matter interfused in the precipitation of calcite during the bacterial sulphate reduction.

Table 1 Summary statistics of microthermometric measurements in fibrous calcite in black shale from Es4s–Es3x interval. Well

Depth (m)

n

Th (°C) Min − max

Tm (°C) Min − max

Eq.wt% NaCl

Mineral

Min − max

⎡ ⎤ ⎣ Mean ± S. D. ⎦

⎡ ⎤ ⎣ Mean ± S. D. ⎦

⎡ ⎤ ⎣ Mean ± S. D. ⎦

NY1

3375.10

7

91.4 to 115.0 102 ± 8.35

−7.7 to − 6.2 −7.0 ± 0.50

9.4 to 11.2 10.5 ± 0.59

NY1

3295.35

5

88.7 to 108.4 95.9 ± 7.98

−6.7 to − 4.7 −5.7 ± 0.75

7.6 to 10.1 8.9 ± 0.93

N872

3206.00

7

94.6 to 117.4 103.6 ± 7.89

−6.2 to − 4.6 −5.4 ± 0.60

7.9 to 9.4 8.4 ± 0.86

N872

3206.40

7

86.4 to 112.3 99.0 ± 8.00

−6.1 to − 5.2 −5.7 ± 0.35

8.1 to 9.4 8.8 ± 0.47

fibrous calcite fibrous calcite fibrous calcite fibrous calcite

5.2.2. Constraints from δ18O composition The δ18O values are a function of the oxygen-isotope composition of the source fluid and temperature-dependent oxygen-isotope fractionation between calcite and fluid (Azmy et al., 2008; Boggs, 2009; Dietzel et al., 2009; Zheng, 2011; Azomani et al., 2013; Hou et al., 2016). The similarity of the δ18Ocal of granular calcite with that of host rock suggests that the brown granular calcite precipitated in shallow settings possibly at the same temperature with the lake water. On the contrary, the δ18Ocal values of fibrous calcite are more negative than those of the host rock. This is probably due to precipitation at higher temperatures (Al-Aasm et al., 1993). Fig. 11 demonstrates the calcite-water equilibrium relationship based on temperature (Friedman and O'Neil, 1977). Assuming a precipitation temperature range between 20 °C and 30 °C for the lacustrine sedimentary carbonate in the host rock (Ma, 2017), the parent fluid δ18Ofluid composition is expected to be between −7‰ and −3‰VSMOW, which is close to the earlier suggested δ18O value of the lake water(-5.0%; Yuan et al., 2015). If this value is assumed to represent the oxygen isotopic composition of pore waters for fibrous calcite precipitation, a water temperature range of 33–70 °C is expected, which is clearly in contradiction with timing of fibrous calcite precipitation discussed above. As a result, an altered pore fluid is likely the case. The measured δ18Ocal of the fibrous calcite and their suggested approximate temperature of precipitation from their Th values imply that the δ18Ofluid values of their parent diagenetic fluids were

sulphate reduction. 5.2. Origin of diagenetic fluid 5.2.1. Constraints from δ13C composition The stable carbon isotopes are highly stable in different carbon pools with deep circulation characteristics and therefore can be used to trace carbon sources (Talma and Netterberg, 1983; Siegel et al., 2004; Cao et al., 2018). The high organic matter abundance excludes the impact of meteoric water intrusion, which can provide CO2 rich in 12C (δ13C < -7‰; Aggarwal et al., 2004). Organic matter (δ13C < −25‰) is a huge carbon source, and bicarbonate (HCO3−) with different δ13C signatures would be released in different process of organic matter alteration during burial diagenesis (Irwin et al., 1977). The HCO3− produced from bacterial sulphate reduction almost completely inherited δ13C signatures in organic matter so that a much depleted carbon isotope composition of carbonate cements from this zone is anticipated (Irwin et al., 1977; McLane and Michael, 1995). The methanogenic zone that usually underlies the bacterial sulphate reduction zone in the sediment column has been suggested to release HCO3− with heavy carbon isotope signatures (δ13C ≈ 0–15‰ VPDB; Raiswell, 1987; Wolff et al., 1992). There is evidence that thermal decarboxylation of organic matters could produce heavier (but still negative) CO2 (−8‰ to −23‰VPDB; Sensuła et al., 2006). The

Fig. 8. Hydrocarbon inclusions trapped in fibrous calcite. (A) Hydrocarbon inclusions under transmitted light. Well N872, 3204.7 m. (B) Hydrocarbon inclusions fluorescing blue-white in UV light. Same horizon as (A). (C) Hydrocarbon inclusions under transmitted light. Well NY1, 3375.1 m. (D) Hydrocarbon inclusions fluorescing blue-white in UV light. Same horizon as (C). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 9. (A) and (B) Details of two-phase inclusions in fibrous calcite. Well N872, 3204.7 m. (C) and (D) Details of two-phase inclusions in fibrous calcite. Well NY1, 3295.35 m (E) Histogram of homogenization temperature of two-phase inclusions in fibrous calcite. (F) Scatter diagram of estimated salinity vs. homogenization temperature of two-phase inclusions in fibrous calcite. Table 2 Carbon- and oxygen-isotope compositions statistics of three types of calcite in black shale from Es4s–Es3x interval. Phase

n

δ13C‰(VPDB) Min − max

fibrous calcite

10

granular calcite

4

host shale (micritic calcite)

10

δ18O‰(VPDB) Min − max

⎡ ⎤ ⎣ Mean ± S. D. ⎦

⎡ ⎤ ⎣ Mean ± S. D. ⎦

1.8 to 5.0 3.49 ± 1.13 −0.2 to 4 0.65 ± 0.70 3.7 to 6.3 4.75 ± 0.82

−13.7 to − 11.4 −12.67 ± 0.68 −8.4 to − 7.7 −8.05 ± 0.31 −9.1 to − 7.2 −8.10 ± 0.59

approximately between −1.0‰ and +3.1‰VSMOW, which is consistent with the mesogenetic diagenetic fluid in Eocene sandstones (Han et al., 2012; Guo et al., 2014; Ma et al., 2016; Wang et al., 2016).

Fig. 10. δ13C VPDB versus δ18O VPDB cross-plot of different calcites.

5.3. Veins formation mechanism

fibrous veins: one includes fracturing as an essential step (Ramsay, 1980) and the other proposes that fibres growth takes place at unfractured surface (Durney and Ramsay, 1973; Fisher and Brantley, 1992). In the first model, the opening of the veins is usually considered as a result of overpressure, which develops horizontal cracks as the fluid

The opening mechanisms of bedding-parallel calcite veins have been much discussed because of the special mineral habit and their potential to contain information on kinematics of deformation in rocks (Durney and Ramsay, 1973; Cox, 1987; Urai et al., 1991; Means and Li, 2001). There are essentially two opposite models for the formation of 880

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Fig. 11. Temperature (T) vs. δ18O diagenetic fluid for various δ18O calcite values using equation: 103 ln α = 2.78 × 106T −2 − 2.89 (Friedman and O'Neil, 1977). The δ18O composition of the lake water responsible for the lacustrine sedimentary micritic calcite in the host rock is determined with the assuming precipitation temperature from 20 °C to 30 °C. δ18O value of diagenetic fluid responsible for fibrous calcite calculated from homogenous temperature and the δ18O compositions of fibrous calcites is −1.0‰–+3.1‰VSMOW. FiCal = fibrous calcite, grCal = granular calcite.

Fig. 12. Sketch illustrating the formation process of the fibrous calcite veins as sub-critical crack growth at least partly drived by force of crystallization.

those microstructures do not occur. In the second model, veins may grow continuously due to active opening by the force of crystallization (Fletcher and Merino, 2001; Means and Li, 2001; Wiltschko and Morse, 2001). In such case, the calcite fibres precipitate at the lamination seam without fracturing and push the wall rock back (Wiltschko and Morse, 2001). The cone-in-cone structures are characteristic of continuous growth (Hilgers and Urai, 2005). Earlier studies suggested that no bedding-parallel hydraulic fracture develop until the pressure coefficient reaches 2.5 (considering average

pressure approaches the lithostatic pressure (Zanella and Cobbold, 2011; Cobbold et al., 2013). Under such a condition, calcite will precipitate when fluid pressure drops due to fracturing, then fractures will be sealed and fluid pressure increases again. The process of repeated fracturing and sealing — so called crack-seal mechanism can produce veins occluded by fibrous or elongate crystals with serrated boundaries, as well as linear bands of solid inclusions parallel to the vein wall or solid inclusions trails at high angle to the vein wall indicating opening trajectory (Ramsay, 1980; Cox, 1987; Dunne and Hancock, 1994). However, this is not the case of the currently investigated veins where 881

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Pfc = ln Ω

(RT ) (−ΔV )

(1)

Where Pfc is force of crystallization, Ω is the degree of supersaturation of pore fluid above that in equilibrium with the solid at hydrostatic fluid pressure, R is the gas constant, T is temperature, ΔV is the difference between the molar volume of solid and solutes. Force of crystallization increases with the increasing temperature and degree of pore fluid supersaturation. According to the data from Dewers and Ortoleva (1990), the force of crystallization of calcite is 53.78 MPa at 70 °C (343.15 K) and supersaturation of 2.0 under conditions of equilibrium. The absence of brown granular calcite within some parts of fibrous calcite veins, especially in the cone-in-cone structures (lower part of Fig. 4D and upper part of Fig. 5A), where there is no nucleation surface with the force of crystallization, indicates that force of crystallization is not the only force involved. The initial cracks may exist on the seam between brown granular calcite and wall-rock or within the lamina under the force of fluid overpressure or any other force. The sub-critical crack growth might play significant roles in the continuous growth process of veins. The formation of fibrous calcite indicates a competition might have inhibited crystallization process, which suggests that growing crystals remain in contact with the opposite wall rock at all times and the force of crystallization most likely contributed to the crack growth (Fig. 12). Detailed microstructure observation reveals that sinusoidal solid inclusions in the cone-to-cone structure originated from previous straight lamina (Figs. 3B and 5B) and occurred when several adjacent beef veins grow to form a composite one (Fig. 5D). Accordingly, the gradual differential displacement process of solid inclusions records the process of vein growth. Assuming a total constant growth rate (defined as the sum of growth rates along one vein–wall interface at different locations; Hilgers and Urai, 2005), an antitaxial model is suggested to explain the formation of cone-to-cone structure based on the occurrence of sinusoidal solid inclusions (Fig. 13). In this model, the median lines keep straight as observed in the samples and sub-critical crack growth happened during the veins growth where the force of crystallization contributed to the crack propagation. The essence of sinusoidal inclusions arrangement is that the straight lamina has been displaced at variable distances from its original position due to fluctuations in the growth rate of crystals. In the antitaxial model (Fig. 13), differences in growth rate along the solid inclusion lead to the sinusoidal arrangement of solid inclusions, which show cone-in-cone structure in third dimension. It is believed that a fluctuation of adhesion at the vein wall interface leads to the growth rate difference along the solid inclusion where self-organization of crystal growth may have also some contribution (Hilgers and Urai, 2005). Based on the discussion above, it is possible to reconstruct the scenario of calcite veins in black shale of Dongying Depression (Fig. 14). The story started from lake water-sediment interface to depths of a few hundred meters, where bacterial sulphate reduction (BSR) occurred in soft and organic-rich sediments as follows:

Fig. 13. Sketch illustrating the formation of cone-in-cone structure. In this models, the total growth rate at different locations along the vein-wall interface are same. Two adjacent seeds sandwiched a lamina occurred in black shale. Calcite fibers grow on two side of the upper seed (median line) and one side on the lower seed (median line). From stage 1 to stage 3, as calcite fibers grow, inclusions become more and more curved. The fibers precipitated at the veinwall (solid inclusion) interface, and the crystals becomes younger from median line to the vein-wall (solid inclusion) interface. The fluctuation of the growth rate along the solid inclusion leads to the increasing curvature of inclusions and no intercrystalline sliding happened.

rock density 2.5 g/cm3; Liu and Xie, 2003). Though the overpressure coefficient of Dongying Depression in any stage did not exceed 2.0 (Li et al., 2008; Guo et al., 2010), it is unrealistic to exclude the possibility that overpressure may lead to horizontal cracks in shale for two reasons: (a) most of the overpressure data comes from sandstone, and the fluid pressure in the mudstone is usually higher than that in sandstone (Guo et al., 2010); and (b) the pressure distribution in the mudstone is strongly heterogeneous and local pressure can be much higher (Hunt, 1990). However, the formation process with fracture can't explain the microstructures especially sinusoidal solid inclusions and smooth fibrous crystals in the veins (Bons and Jessell, 1997; Hilgers and Urai, 2005). Force of crystallization is the pressure that a crystal growing in a solution supersaturated to a given degree can grow against or exert on its surroundings (Maliva and Siever, 1988). It can be restricted to equilibrium thermodynamics by equation (Maliva and Siever, 1988; Wiltschko and Morse, 2001):

CH2 O

+ SO4−2 → HS − + 2HCO3− + H+

(2)

4FeOOH + 4SO42 − + 9CH2 O → 4FeS + 9HCO3− + 6H2 O + H+ 2Fe2 O3

+

8SO42 −

+ 15CH2 O → 4FeS2 +

15HCO3−

(3)

+ 7H2 O + OH− (4)

Bicarbonate generation from biodecomposition of organic matter and increase in alkalinity by reactions (3) and (4) enhanced carbonate precipitation in BSR. The granular calcite precipitated homogeneously in the massive mudstone or along the laminas in the fissile shale. The mixing of carbon derived from organic matter and lacustrine pore waters can explain the depleted δ13C signature of granular calcite (Fig. 14). As soon as sulphate was totally reduced in the BSR zone, microbial methanogenesis started and continued with progressive burial until 882

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Fig. 14. Schematic sketch illustrating the carbon source of bedding-parallel calcite veins. OM, organic matter; BSR, sulphate-reducing bacteria.

water during the early stage of the “oil window” on the basis of granular calcite median line. The sub-critical crack growth possibly played a role in opening veins, where force of crystallization contributed to the crack propagation, and the cone-in-cone structures formed when several adjacent beefs grew together. The δ18Ofluid values of parent diagenetic fluids of calcite fibers were approximately between −1.0‰ and +3.1‰ VSMOW and suggest an enrichment of 18O during the black shale burial. The carbon isotope composition of the parent bicarbonate solution of the fibrous calcite originated from a mixed source of inorganic carbon (from previous carbonate dissolution) and organic carbon (from fermentation and thermally decarboxylation), with a moderate δ13C signature from +1.8‰ to +5.0‰ (+3.49 ± 1.13‰) VPDB. The results provide a new explanation for the origin of the bedding bedding-parallel fibrous calcite veins and implications for better understanding the organic-inorganic interactions during the black shale diagenesis and the formation mechanism of shale oil reservoir in lacustrine basins.

temperature reached ∼75 °C, where bacterial activity was terminated (Morad, 2009). Methanogenesis (Me) is believed to occur by the fermentation of simple organic compounds as follow:

2CH2 O → CH4 + CO2

(5)

The CO2 with positive carbon isotopic values was released in this process (Irwin et al., 1977). In the internally buffered carbonate system, pH may decrease due to increased PCO2, leading to carbonate dissolution (Surdam et al., 1984). Bicarbonate with intermediate δ13C inherited its composition from the mixed carbon derived from different sources. The pH of the carbonate system is externally buffered by the carboxylic acid anions at the early stages of “oil window”. As a result, the decarboxylation of organic matter with high and increasing PCO2, would enhance the precipitation of carbonate cements (Surdam et al., 1984). The calcite preferentially precipitated from the supersaturation pore fluid mostly along the previously formed granular calcite (acting as seeds) because of its low adhesion with host rock. In the overpressure cell, the growth of calcite fibres wedged off the wall rock with force of crystallization and the bedding-parallel fibrous calcite veins grew.

Acknowledgements

6. Conclusions

This work was supported by Natural Science Foundation of China (Grant No. 41802172), National Science and Technology Major Project, P.R. China (Grant No. 2017ZX05009-001), China Postdoctoral Science Foundation (Grant No. 2018M632742) and Natural Science Foundation of Shandong Province (Grant No. ZR2018BD014). We are thankful to Analytical Laboratory of BRIUG for carbon and oxygen analysis and

The bedding-parallel fibrous calcite veins that exhibit antitaxial patterns grow over two stages: (1) the granular calcite precipitated in a BSR zone with relatively depleted δ13C signature and occurred as median line; and (2) the fibrous calcite precipitated from salty pore 883

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technical support.

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